PROMISING HYDROCARBON POTENTIAL SEEN IN CANNING BASIN OFF AUSTRALIA

Virginia L. Passmore
Bureau of Mineral Resources
Geology & Geophysics
Canberra

The Canning basin is a large Phanerozoic sedimentary basin on the northwest coast of Western Australia (Fig. 1) and is comparable in size to Texas.

The western one third of the basin is offshore. The Australian federal government and West Australian state government have released nine large vacant areas, encompassing a total of 1,420 blocks in the offshore part of the Canning basin, in the May 1991 Release of Offshore Petroleum Exploration Areas (OGJ, June 10, p. 24).

The vacant areas being offered, Areas W91-2 through W91-10 (Fig. 2), cover most of the offshore part of the basin except for the Rowley Shoals region, a present day reef area over which a marine park has been proposed.

The vacant areas are large, ranging in size from 128 to 192 graticular blocks (a single graticular block covers an area 5 min by 5 min). Water depths in the release areas vary from shallow near shore in Areas W91-5, W91-6, and W91-10 to slightly deeper than 1,000 m in Area W91-2.

The Canning basin lies among a group of prolific hydrocarbon basins that underlie Australia's Northwest Shelf.

The Northwest Shelf region1 is a geographic province that covers the Browse basin and offshore parts of the northern Carnarvon, the Canning, and the Bonaparte basins (Fig. 1).

Within this region are major Mesozoic oil and gas accumulations2 that contain the majority of Australia's gas reserves and a significant amount of its oil. This paper briefly discusses the petroleum potential of the offshore Canning basin by looking at the basin's structural and tectonic evolution (primarily its offshore development) and its depositional history with respect to hydrocarbon relevant aspects such as traps, reservoir, and source.

EXPLORATION HISTORY

The Canning basin has a long history of exploration.3 More than 200 wells, mainly exploration and stratigraphic holes, have been drilled in the basin; only 13 of the wells are offshore.

Unlike in neighboring Northwest Shelf basins, Canning basin exploration activity has been primarily on land. To date, the Canning basin has eight small discoveries (Fig. 3), all onshore. All of these hydrocarbon accumulations are in Paleozoic reservoirs.4 Blina, the first discovery and the largest of the onshore accumulations, was not discovered until 1981.

There are as yet no oil or gas discoveries offshore in the Canning basin, but 1 Perind; in the Fitzroy graben and 1 Phoenix in the Bedout subbasin recorded minor oil staining in Permian and Triassic sediments, respectively (Fig. 3). By most standards, the offshore is still sparsely explored.

Fewer than 80 seismic surveys have been shot over this part of the basin, which covers an area nearly 200,000 sq km.

Drilling operations have tested relatively little of the offshore region.

Twelve of the 13 offshore wells are on the continental shelf, clustered in the Bedout sub-basin and the Fitzroy graben with only one well sited on the upper continental slope in the Rowley sub-basin (Fig. 3).

STRUCTURAL SETTING

The offshore structural features of the Canning basin are shown in Fig. 4.

The northwesterly Paleozoic structural grain that defines the basin's onshore features continues offshore onto the continental shelf.

The basin takes on a northeasterly trend farther west in the southern reaches of the continental shelf and on the slope.

This northeasterly trend was superimposed on the older structural grain by the rifting and breakup of Gondwana in the Mesozoic.

The Leveque and Pilbara shelves mark the basin's northern and southern offshore limits, respectively. The Leveque shelf separates the Canning from the Browse basin to the north, and the North Turtle arch and the northerly trending fault north of the arch mark the boundary between the Canning and the Carnarvon basin to the south.

Within the Canning basin, the northwest trending Broome arch, an old basement high, separates the Fitzroy graben in the north from extensions of the southern onshore Paleozoic subbasins and depressions and the Bedout sub-basin, a Permo-Triassic depocenter. Seaward of the continental shelf and these features lies the Rowley sub-basin, a Mesozoic and Tertiary depocenter.

STRUCTURAL/TECTONIC EVOLUTION, POTENTIAL TRAPS

The Canning basin has been an active sedimentary basin during most of Phanerozoic.

Seismic survey data5 6 and well data on and offshore suggest that the basin has gone through several tectonic episodes since its formation in early Ordovician.7 8 9

The Paleozoic episodes controlled sedimentation and trapping on and offshore. Discussions of structural development prior to the Alice Springs orogeny in the Carboniferous are based on onshore data due to the lack of information on the evolution of Paleozoic structures offshore.

As the onshore Paleozoic features appear to extend onto the continental shelf without a major break, it has been assumed that the offshore part of these features has had a Paleozoic development that is similar to that determined for the onshore parts.

PALEOZOIC

The Canning basin formed initially as an intracratonic downwarp.

Syndepositional faulting across the Broome arch and along its flanks produced horst blocks, grabens and half-grabens, and potential structural traps. Stratigraphic traps may have developed offshore on topographically higher basement features, such as the Broome arch, which have attracted carbonate buildups onshore.

Intracratonic rifting in the Devonian formed the Fitzroy graben, a series of half grabens in the northern part of the basin. This asymmetrical trough is cut by transfer faults along which the hinged and faulted margins of the trough are switched,10 forming a number of terraces along both margins. Most of the onshore discoveries, both stratigraphic and structural plays, are sited on the terraces that flank the onshore part of the Fitzroy graben (Fig. 3).

Late Carboniferous faulting associated with the Alice Springs orogeny formed a series of horsts and faulted blocks that are the main structural traps in the offshore Fitzroy graben (Fig. 5).

The Bedout high in the Bedout sub-basin may also have formed in the Carboniferous.11 An emergent feature until the late Triassic, the flanks of the high may be prospective for stratigraphic traps within the Permian and Triassic sediments that thin onto its flanks.

Major erosion associated with the orogeny removed much of the Paleozoic sequence over the southwestern region of the onshore part of the basin and probably over the adjacent parts of the offshore. On the Broome arch, erosion cut down into the Ordovician sequence. The extent of the erosion offshore is not known.

A later cycle of erosion in the late Permian and Triassic peneplained much of the late Paleozoic sequence within the offshore Fitzroy graben.

MESOZOIC

Incipient rifting on the Northwest shelf in the Permian to early Triassic preceded Jurassic breakup of Gondwana and created a northeasterly structural grain over much of the outer offshore Canning basin.

Rifting produced block faulting in the Bedout and Rowley sub-basins and the area west of the Leveque shelf (Fig. 6). Permian igneous intrusions into sediments in the Fitzroy graben and on the terraces, and volcanic activity on the Bedout high are probably related to this tectonic activity.

Some of the Permo/Triassic faults were reactivated as growth faults in the late Triassic to early Jurassic in association with wrench movement in the basin.9 These faulted blocks became topographic highs for the development of Triassic reefs (Fig. 7), and the formation of anticlinal drape traps.

Reactivation of faults and differential compaction over buried fault blocks have created rollover structures and the stacking of potential reservoirs (Fig. 8) for both fault block traps and anticlinal traps.

Limited post-Jurassic faulting is found in the western part of the offshore Canning basin, but there is little development of the late Jurassic and early Cretaceous fault traps that are prime areas of hydrocarbon entrapment in other Northwest Shelf basins.

Seismic data suggest that stratigraphic traps may be developed in the deeper water parts of the Rowley subbasin, such as the Neocomian (early Cretaceous) reef complex above an older buried horst in the Mermaid reef area (Fig. 7).

DEPOSITIONAL EVOLUTION, PROSPECTIVE RESERVOIR, SOURCE ROCKS

Well data on and offshore and paleogeographic environmental reconstructions over the Northwest Shelf area12 suggest that the offshore Canning basin contains sediments ranging in age from early Ordovician to Recent (Fig. 9).

Little, however, is known about the Paleozoic sediments offshore, as few offshore wells penetrated below the Mesozoic sequence. Wells drilled in the Fitzroy graben did drill into older rocks, but even these wells stopped in Devonian or younger Paleozoic units.

The single well in the Rowley sub-basin bottomed in Lower Jurassic sediments. The offshore stratigraphy has been described by numerous authors.8 13 14

Descriptions of the offshore stratigraphy has been established from offshore well data and extrapolations from onshore wells.

The main Paleozoic source and reservoir rocks onshore are also believed to be prospective offshore. Offshore drilling and seismic have identified additional source and reservoir rocks in Mesozoic sequences (Fig. 9). Paleogeographic reconstructions and well data indicate that potential source and reservoir rocks were likely to have been deposited over most parts of the offshore Canning basin.

ORDOVICIAN

It is highly probable that the widespread Ordovician epeiric sea conditions onshore also existed in the present offshore.

Carbonates, similar to those on the onshore Broome arch and northern flank of the Fitzroy graben which are the main onshore Ordovician exploration targets, were probably deposited on the topographically higher offshore features, while more clastic sediments were deposited in the adjacent troughs.

Onshore, the topographically higher basement features, such as the Broome arch, that attracted carbonate buildups were also favorable for diagenetic alteration and development of secondary porosity.

Similar reservoir enhancement could also be expected offshore. The organic-rich shale (Goldwyer formation) that is the source rock for the onshore Ordovician would have also been deposited offshore.

Major pre-Permian erosion cut down into Ordovician sediments on the Broome arch and depressions south of the arch, removing much of the sequence. Provided it has been preserved, Ordovician source rocks may be a significant hydrocarbon source for the present shallower offshore Paleozoic traps.

There is little onshore data from which to extrapolate the offshore post-Middle Ordovician to late Devonian sedimentary history. The sediments are most likely to be mainly marine clastics and carbonates.

The contraction of the Devonian sea that resulted from apparent uplift of part of the southern Canning basin in the mid-Devonian restricted offshore Devonian sedimentation to the newly formed rift valley that marked the formation of the Fitzroy graben and to a postulated arm of the sea farther west that connected the Carnarvon basin with the other Northwest Shelf basins.

Organic-rich basinal shales, similar to those that sourced Blina field, were probably deposited in the off shore Fitzroy graben. The type of marine sediments deposited farther west is unknown.

Limestone reefs developed on the terraces flanking the Lennard shelf, but it is not known whether reefs also grew along the edges of the graben further west. Both the Leveque shelf and the Broome arch were apparently emergent at this time.

Reef plays similar to Blina field could be present on the terraces flanking these structures if porous reefal carbonates were deposited.

Carboniferous sedimentation was also restricted to the rift valley. As the seas became shallower in the Carboniferous, shallow marine carbonates and lagoonal clastic sediments, which are reservoir, source, and seal for several small onshore fields, were deposited in the Fitzroy graben. Offshore wells have intersected clastics in the Lower Carboniferous section.

SOURCE ROCKS

Hydrocarbon discoveries in the Canning basin are sourced by onshore Ordovician, Devonian, and Carboniferous source rocks.

Offshore, Lower Carboniferous source rocks are presently within the oil window in much of the offshore Fitzroy graben, but they are likely to be overmature farther west. Devonian and Ordovician source rocks offshore have not been sampled, but generated hydrocarbons are likely to have already migrated into Paleozoic traps.

On the Broome arch, onshore Ordovician source rocks appear to have reached maturity in the Paleozoic prior to Devonian sedimentation.15

By the early Permian, cold climate marine conditions had spread over most of the basin. The marine sandstones and shales that blanketed the basin were deposited unconformably on basin sediments as old as early Ordovician.

Early Permian clastics are reservoir and seal for several of the onshore fields and are expected to have similar potential offshore. Where they have been intersected in the offshore Fitzroy graben, the early Permian sandstones have good porosity.

None of the offshore wells in the Bedout sub-basin have penetrated below Upper Permian volcanics; therefore reservoir, source, and seal potential are unknown over most of the offshore part of the basin. It is expected that the reservoir potential will probably be limited in the western parts of the offshore where the sandstones are more deeply buried.

MESOZOIC

Marine regression in the Triassic confined Triassic deposition to the Bedout and Rowley sub-basins and the area west of the Leveque shelf.

Interbedded fluvial and paralic sands and shales that form potential Triassic reservoir, source, and seal were deposited. In the deeper water areas of the basin to the west, regional reconstructions suggest that marine carbonates were deposited. Late Triassic reefs formed above some of the horst blocks farther west in the present deepwater areas.16

Deposition resumed over all the offshore Canning in the early Jurassic, burying emergent Carboniferous and Permian fault blocks as well as younger fault blocks which formed when some of the previous faults were reactivated (Fig. 6). Fluvial and deltaic clastics covered the present shelf areas by Middle Jurassic, providing reservoir, source, and seal.

Marine shales and siltstones were restricted to the present deepwater areas until continental breakup in the Middle Jurassic and major transgression of the sea in the late Jurassic. The main depocenter shifted west in the Jurassic to the Rowley sub-basin.

In the Bedout sub-basin and the eastern Rowley subbasin, early to middle Triassic source rocks are presently within the oil window. However, the Triassic is likely to be overmature in the deeper water areas.

On the limited data available, early and middle Jurassic source rocks appear to have just entered the mature zone in the eastern Rowley sub-basin. In the deeper water areas the Lower Jurassic and possible middle to late Jurassic source rocks may be presently generating hydrocarbons as are Jurassic source rocks in the other Northwest Shelf basins.

The main source rocks for most of the Northwest Shelf oil are the middle and late Jurassic shales. These shales were deposited in restricted deepwater troughs that provided conditions that were favorable to the accumulation of organic-rich shales.2

Deep water Jurassic troughs developed on the present continental shelf and upper slope of the Browse, Carnarvon, and Bonaparte basins, but so far similar troughs have not been recognized in the offshore Canning basin.

SUMMARY

The offshore part of the Canning basin appears to have significant hydrocarbon potential. Source, reservoir, and seal occur in both Paleozoic and Mesozoic sequences, and plays can be recognized over much of the offshore.

The most common plays in both the Rowley and Bedout sub-basins are either fault traps or anticlinal drape traps. Paleozoic fault block and horst traps are the only recognized plays in the Fitzroy graben. If porous reefs can be found on the flanks of the offshore Fitzroy graben a stratigraphic play type can be added to this area.

Many of the source and reservoir rocks in the Canning basin are older than those in the other Northwest Shelf basins. Successful explorationists in the offshore Canning basin will need to direct their efforts towards different targets than those traditionally tested on the Northwest Shelf.

ACKNOWLEDGMENTS

The author wishes to thank Japan National Oil Corp. for permission to use illustrations and seismic lines. Thanks also to Bureau of Mineral Resources colleagues Barry West, Ian Lavering, and Geoff O'Brien, who reviewed this paper; Dennis Taylor of CRA Exploration Pty. Ltd. for his helpful comments; and the BMR cartography section for preparing the figures. Publication is by permission of the Director, Bureau of Mineral Resources, Geology and Geophysics.

REFERENCES

1. Bradshaw, M.T., Yeates, A.N., Beynon, R.M., Brakel, A.T., Langford, R.P., Totterdell, J.M., & Yeung, M., Palaeogeographic evolution of the Northwest Shelf region. In: Purcell, P.G. & R.R. (Eds), The Northwest Shelf, Australia, proceedings of the Petroleum Exploration Society of Australia Symposium, Perth, W.A., 1988, pp. 29-54.

2. Purcell, P.G. & R.R. (Eds), The Northwest Shelf, Australia. Proceedings of the Petroleum Exploration Society of Australia Symposium, Perth, W.A., 1988.

3. Passmore, V.L., & Towner, R.R., A history of geological exploration in the Canning basin, Western Australia. Earth Science History, 6(2), 1987, pp. 159-177.

4. Goldstein, B.A., Waxings and wanings in stratigraphy, play concepts and perceived prospectivity in the Canning basin. The APEA Journal, 29(l), 1989, pp. 466-508.

5. Japan National Oil Corp., Geological and geophysical study of the offshore Canning basin in the Northwest Shelf of Australia, unpublished company report, 1988.

6. JNOC, Geological and geophysical study of the northeastern part of the offshore Canning basin on the Northwest Shelf of Australia, unpublished company report, 1989.

7. Brown, S.A., Boserio, I.M., Jackson, K.S., & Spence, K.W., The geological evolution of the Canning basin-implications for petroleum exploration. In: Purcell, P.G. (Ed), The Canning basin, W.A., proceedings of the Geological Society of Australia and Petroleum Exploration Society of Australia Symposium, Perth, 1984, pp. 85-96.

8. Horstman E.L. & Purcell, P.G., The offshore Canning basin - a review. In: Purcell, P.G. & R.R. (Eds), The Northwest Shelf, Australia. Proceedings of the Petroleum Exploration Society of Australia Symposium, Perth, W.A., 1988, pp. 253257.

9. Warris, B.J., Canning basin, off-shore. In: Leslie, R.B., Evans, H.J. & Knight, C.L. (Eds), Economic Geology of Australia and Papua New Guinea. 3. Petroleum. Australian Institute of Mining and Metallurgy, Monograph 7, 1986, pp. 185-188.

10. Drummond, B.J., Etheridge, M.A., Davies, P.J., & Middleton, M.F., Half-graben model for the structural evolution of the Fitzroy Trough, Canning basin, and implications for resource exploration. The APEA Journal, 28(1), 1988, pp. 76-86.

11. J. Bradshaw, BMR, personal communication.

12. Bureau of Mineral Resources Paleogeographic Group, Australia: Evolution of a continent, 1990.

13. Forman, D.J. & Wales, D.W. (Compilers), Geological evolution of the Canning basin, Western Australia. Bureau of Mineral Resources, Australia, Bulletin 210, 1981.

14. Forrest J.T. & Horstman, E.L., The Northwest Shelf of Australia geological review of a potential major petroleum province of the future. In: Halbouty, M.T. (Ed), Future petroleum provinces of the world. Proceedings of the Wallace C. Pratt Memorial Conference, Phoenix, 1984, AAPG Memoir 40, 1986, pp. 457-486.

15. D. Taylor, CRA Petroleum Pty. Ltd., personal communication.

16. Falvey, D.A., Symonds, P.A., Colwell, J.B., Willcox, J.B., Marshall, J.F., Williamson, P.E., & Stagg, H.M.J., Australia's deepwater frontier petroleum basins and play types. The APEA Journal, 30(1), 1990, pp. 238-262.

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